1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and, more particularly, to an electrographic layer which contains an acetal resin having a specific structure. The present invention also relates to an electrophotographic apparatus, a device unit and a facsimile machine which employs the electrophotographic photosensitive members.
2. Description of the Related Art
Most known electrophotographic photosensitive members employing organic photoconductive substances have photosensitive layers formed by dispersing charge generating substances of relatively low molecular weights, such as azo pigments or phthalocyuanine pigments, in suitable binder resins. One kind of organic electrophotographic photosensitive member has a so-called laminated-type photosensitive layer comprising two layers: a charge generating layer containing a charge generating substance; and a charge transporting layer containing a charge transporting substance. Such an organic electrophotographic photosensitive member exhibits good sensitivity, potential characteristic and durability, and therefore is widely used.
In such electrophotographic photosensitive members, the characteristics of the photosensitive member are substantially determined by factors such as carrier generating efficiency, carrier transporting efficiency of the photosensitive layer and further, in the laminated type, the efficiency in injecting carriers from the charge generating layer into the charge transporting layer. It is speculated that these factors are affected by the characteristics of a binder resin as well as the characteristics of both charge generating substances and charge transporting substances. However, the primary purpose of the development of the binder resin has been to improve the binding characteristic, the pigment dispersing characteristic and the mechanical strength thereof. Although Japanese Patent Application Laid-Open Nos. 62-30254 and 62-95537 describe that the structure and molecular weight of the binder resin also affect the electrophotographic characteristics such as sensitivity, durability or residual potential of the photosensitive member, they do not clearly describe that a binder resin can serve as a functional resin, and satisfactory electrophotographic characteristics have not been achieved so far.
To meet growing demands for further improving image quality and durability, an electrophotographic photosensitive member having better electrophotographic characteristics must be developed.